1. Field of the Invention
This invention relates to the structure of a centrifugal blower wheel with backward curved blades, and more specifically concerns a wheel with blades having airfoil cross sectional shapes. This type of centrifugal blower is used in exhausting, ventilating and air conditioning devices.
2. Prior Art
It is well known that the centrifugal blower wheel (hereinafter referred to as wheel) with airfoil shaped blades (hereinafter referred to as blades), is highly efficient and low in noise.
This type of wheel comprises a shroud, a disk and blades. Conventional wheels are classified, roughly, into three types based on the difference of material and manufacturing method. Hereinafter, these wheels will be referred to as type A wheel, type B wheel and type C wheel.
The type A wheel comprises the airfoil blades which are made of steel or aluminum sheet by press working, and the shroud and the disk which are made of the same metal as the blades. The blades are fixed to the shroud and the disk by means of caulking or the like. It is difficult to form the blades precisely into an airfoil shape by press working, therefore, the type A wheel is inferior to the other type wheels in efficiency, and higher in operating noise caused by air turbulence on the surface of the blades. Furthermore, metal has a high characteristic frequency, and thus higher operating noise. However, the type A wheel is the lightest in weight and the most rigid as a rotating device among these three types of wheels.
The type B wheel comprises the blades which are made of aluminum by extrusion molding, the shroud and the disk which are made of aluminum sheet. The blades are fixed to the shroud and the disk by means of welding or the like. Since the blade of this type wheel has the most accurate airfoil shape, the type B wheel is the most efficient but also the heaviest.
The type C wheel comprises the airfoil blades which are made of plastic material by injection molding, the shroud and the disk which are made of plastic material. The blades are fixed to the shroud and the disk by means of adhering or the like. The type C wheel is more efficient than the type B wheel and the quietest in operating noise among the aforementioned three types of wheels. However, the type B wheel can be deformed easily by heat.
As described above, all three types have their respective advantages and disadvantages, hence no blower wheel is completely satisfactory for all operating environments. Therefore, the type of wheel was chosen considering the blower operating environment.
Recently, more blowers for air conditioning devices have been required to operate with higher efficiency and less noise. At the same time, there is a need for greater compactness, which means the blower must be operated at even higher speeds. Thus, blower wheels have been required which have higher rigidity and dynamic balance in order to withstand the loads due to high speed running. Furthermore, in the case of air conditioning apparatus, more units are being installed in ceilings so as to enable more effective use of floor space. This creates a greater need for blowers to be more lightweight and quieter in operation. However, these requirements cannot be achieved absolutely by the aforementioned conventional wheel structures.